Thermosetting resin reinforcement technology as practiced today for the open-mold fabrication of hot tubs, bathtubs, recreational vehicle components, marine craft and components, etc. is fundamentally unchanged from that of forty years ago. Resin reinforcement is applied to the surface or cosmetic layer, in order to provide essential mechanical properties such as tensile strength, flexural strength, impact strength, and toughness. Thermosetting materials that can function as the reinforcing substrate include unsaturated polyesters, epoxies, polyurethanes, phenolics, vinyl esters, polyureas, polyisocyanurates, and the like, and/or combinations of the aforementioned materials. Combinations of two or more thermosetting chemistries are commonly referred to as interpenetrating networks or hybrid resin systems, the two types being differentiated by the type of reaction chemistry that takes place. Despite improvements in unsaturated polyester resin technology and the advent of hybrid resins, these types of systems have not progressed into an optimal rigidizing technology due to their fundamental dependence on a reactive diluent, such as styrene monomer. Isocyanate-based systems that do not require the use of a reactive diluent have been introduced in an attempt to overcome the drawbacks of conventional rigidizing systems. Unfortunately, these systems have not been able to fulfill the requirements of a rigidizing system in the majority of applications. In some applications, isocyanate-based systems can be used in conjunction with a thermoplastic surface layer of substantial thickness to produce a product with sufficient mechanical properties. However, this approach is not ideal and/or not appropriate for most applications. Consequently, the preponderance of applications requiring rigidizing such as those previously referenced and other applications associated with open-molding, rely almost exclusively upon the use of unsaturated polyester or hybrid resin technology.
In the majority of the prior art applications the surface or cosmetic layer of choice is a clear or pigmented gel coat that is also based on unsaturated polyester resin technology that incorporates the previously noted reactive diluent(s). In some applications the cosmetic layer is formulated as a gel coat using polyurethane technology. However, this technology has yet to find wide spread acceptance owing to its significantly higher cost. Other thermosetting resins that could be incorporated to provide the cosmetic surface include vinyl esters, alkyds, polyurethanes, polyureas, polyimides, epoxy resins, phenolics, amino resins, and allyl resins. In the remainder of the open-molding applications the substrate is a thermoformed thermoplastic polymer that has been incorporated into the component design to overcome some of the inherent deficiencies of the gel coat while additionally providing a high-gloss surface and acceptable appearance. Thermoplastic polymers are those resins that can be processed thermally to produce useful items and include but are not restricted to, polymethylmethacrylate polymers, polyvinyl halides, olefin polymers, styrenic polymers, polyesters, nylons, polysulfones, polycarbonates, polyacetals and the like. Composites, blends, and alloys of the aforementioned thermoplastic resins may also be used as the cosmetic layer. Examples include but are not restricted to polycarbonate/polymethylmethacrylate, polycarbonate/acrylonitrile-butadiene-styrene terpolymers, polycarbonate/polybutylene terephthalate, polystyrene/polyphenylene oxide, acrylonitrile-butadiene-styrene/polybutylene terephthalate, polyurethane/acrylonitrile-butadiene-styrene terpolymers, and the like. It is in conjunction with thermoplastic polymers that most of the hybrid resin-based systems or isocyanate-based systems are utilized as previously noted.
The unsaturated polyesters used in rigidizing systems are typically, but not exclusively, condensation polymers prepared from unsaturated di- or polycarboxylic acid(s) or anhydrides(s) with an excess of glycols and/or polyhydric alcohol(s) that result in a polyester polyol having at least one ethylenically unsaturated group per molecule having predominantly hydroxyl-terminated end groups. Typically the diacids of choice are maleic acid (anhydride), orthophthalic acid (phthalic anhydride) or isophthalic acid, or a combination thereof, with the glycol component being ethylene glycol, diethylene glycol, propylene glycol, neopentyl glycol, or a combination thereof. The resultant polyester polyol in turn is dissolved in an ethylenically unsaturated monomer solution at a level of 30-90 wt. %. Most often the monomer solution of choice is styrene. In addition, unsaturated polyesters can result from the synthesis of an addition polymer that is further modified by incorporation into a condensation polymer. This process typically incorporates maleic acid and dicyclopentadiene to create a diene-terminated ester. The resultant ester is then reacted with one or more of the aforementioned diol(s) that in turn is dissolved in an ethylenically unsaturated monomer solution. Optionally, fillers, fibers, catalysts, promoters, pigments, flame retardants, processing aids such as thixotropic agents and internal lubricants or surfactants, all of which are well known to those skilled in the art, can be added or employed to gain the desired reaction rate(s) and physical properties. The unsaturated polyesters used as gel coats are typically, but not exclusively, based on the same technology as described above while having a lower initial viscosity to facilitate the addition of pigment at various loadings. In order to achieve the desired high-gloss and surface appearance associated with gel coats they are formulated to have a very hard and therefore, brittle surface.
Hybrid systems are typically saturated or unsaturated polyester-polyurethane resins that are well known in the art of thermoset compositions. These resins are normally tougher than unsaturated polyesters and stronger, stiffer and less expensive than polyurethane. Such resins generically comprise a hydroxyl-terminated unsaturated polyester polyol, an ethylenically unsaturated monomer, and a multifunctional isocyanate. Typically, these resins are provided as a two or more component system. Common terminology in the art is to refer to these as an "A-Side" component, containing the multifunctional isocyanate and usually one or more free radical initiators, and a "B-Side" component usually containing the hydroxyl-terminated polyester polyol and ethylenically unsaturated monomer, as well as a polyurethane catalyst, a peroxide promoter, chain extender and optionally water. Examples of typical prior art hybrid systems are set forth in U.S. Pat. Nos. 5,153,261; 5,296,544; 5,296,545; 5,302,634; 5,344,852; 5,447,921; 5,464,919 and 5,482,648. These and other patents cited herein are incorporated by reference.
Various isocyanate-based systems can provide reinforcement to thermoformed thermoplastic components as cited in U.S. Pat. Nos. 4,738,989; 4,748,192; 4,748,201; 4,844,944; 5,380,768 and 5,420,169. The referenced systems can be closed-cell foams, open-cell foams or of the non-foaming type. Typically, but not exclusively, the type of polyol that is incorporated to form the polyurethane network can differentiate these foams. Incorporating typical polyether and/or polyester polyols can produce other rigid foam systems having good properties when multifunctional polyols and/or highly rigid polyols are preferentially used. Examples of prior art in this field can be found in U.S. Pat. Nos. 4,581,388; 5,284,882; 5,496,496 and 5,770,635. The aforementioned hybrid systems in comparison use an unsaturated polyester polyol or an acrylate containing hydroxyl compound to form a crosslinked urethane backbone offering very good properties when combined with a reactive monomer such as styrene monomer or methyl methacrylate monomer. An important consideration must also include the discussion of foam density since it is known that low-density rigid foams have an increased tendency towards shrinkage. This issue is typically addressed by incorporating crosslinking agents and/or highly functional low molecular weight polyols. Their incorporation increases the crosslinking density while creating a pronounced increase in foam strength and a corresponding reduction in shrinkage.
While the above referenced chemistries and technologies represent the prior art as is presently practiced in the industry, it is well known that each exhibits various deficiencies that present an opportunity for improvement. The deficiencies can be categorized into topics that are best separated by their respective chemistries and are addressed accordingly in greater detail in the subsequent discussion.
Unsaturated polyesters as previously noted, are typically condensation polymers prepared from unsaturated di- or polycarboxylic acid(s) or anhydride(s) with an excess of glycols and/or polyhydric alcohol(s). Maleic acid (anhydride) is typically common to these systems as it serves as the reactive center of the polyester alkyd and provides the necessary double bonds for the vinyl polymerization process. By varying the maleic content in the polyester alkyd the reactivity, peak exotherm, chemical resistance to acids and bases, heat distortion temperature and glass transition temperature can be altered to achieve a predetermined formulation criterion. Systems containing orthophthalic acid (phthalic anhydride) have as their drawbacks low chemical resistance and are prone to water absorption. Systems based on isophthalic acid are typically more expensive than orthophthalic acid containing unsaturated polyesters owing to the increased need for energy to incorporate the diacid into the polymer backbone. While offering good hydrolytic and color stability, they typically are deficient in the area of wet-out potential when incorporating a high loading of inert fillers. Diol selection also plays an important part in the physical characteristics of the unsaturated polyester. Propylene glycol and ethylene glycol both are known to be process control critical during the condensation reaction. Propylene glycol in particular is subject to side reactions during the formation of the polyester alkyd while the incorporation of ethylene glycol can result in low strength due to increased rigidity. While the incorporation of diethylene glycol into the polymer back bone typically results in the optimization of the flexural and elongation properties of the polyester alkyd it is prone to having poor hydrolytic stability and a low heat distortion temperature. Dicyclopentadiene containing polyesters typically exhibit higher exotherms that can translate into structural deficiencies as a result of internal thermal stress, and have been observed to exhibit an increase in brittleness. Common to all of these systems is the incorporation of styrene monomer that provides highly reactive double-bond sites essential to the vinyl polymerization process while dissolving the polyester alkyd to reduce the material's viscosity for ease of handling and application. However, while styrene monomer is the predominant reactive diluent used in the industry it has come under significant regulatory scrutiny. Styrene monomer as a singular component has poor physical properties when polymerized. Aside from the aforementioned process control issues that are inherent in condensation polymer chemistries of this nature is the need to inhibit the resultant polymer with free radical scavengers to prevent premature gelation from exposure to heat during processing and to prevent the activation of the polymerization process during storage. While a predetermined amount of the inhibitor(s) is sufficient in slowing the self-polymerization process over a short period of time the storage stability is limited, typically to a period of 90-180 days. The necessity of the inhibitor(s) also dictates the use of higher catalyst levels to facilitate free radical initiation that is caused by the inhibitor's tendencies to retard polymerization initiation under actual application conditions. Styrene monomer itself must also be inhibited to prevent premature polymerization that, in turn, necessitates the use of higher catalyst levels in the unsaturated polyester formulary.
The catalyst selection process is typically dependent upon the type of fabrication to be utilized and the desired rate of reactivity under predetermined operating conditions to include temperature, humidity, required wet properties, the need to facilitate fiber alignment, and to address other handling issues. Ambient temperature systems typically use methyl ethyl ketone peroxides or benzoyl peroxide whereas elevated temperature systems employ but are not limited to, such catalysts as tertiary butyl perbenzoates, tertiary benzyl peroctoates or benzoyl peroxide. Some systems have been formulated to use a combination of catalysts in particular an ambient type combined with an elevated temperature catalyst, to achieve the required reactivity and degree of cure. The term "cure" or "curing" means the transformation of the unsaturated polyester resin composition from a liquid or flowable paste to a solid cross-linked material at the time of application. In open-molding applications the catalyst choice is predominately of the ambient variety. The decomposition of methyl ethyl ketone peroxide occurs in the presence of heat or active metal salts of organic compounds whereas benzoyl peroxide decomposes in the presence of heat or amines thereby giving those practiced in the art a greater latitude in formulating a catalyst system to achieve the desired gel and cure characteristics. To their detriment, the methyl ethyl ketone peroxides are classified by the National Fire Protection Association as a Hazard Class III material thereby requiring special handling and safety precautions. Benzoyl peroxide is classified as a Hazard Class IV and while requiring fewer precautions compared to methyl ethyl ketone peroxide, still requires handling and safety precautions. Emulsions of benzoyl peroxide produce a more stable material, however the resultant benefit is offset by a higher cost. Typically catalysts of this type require an activation energy provided through heat or other means to initiate and accelerate the crosslinking reaction of an ethylenically unsaturated monomer solution, most commonly styrene.
Styrene monomer along with its derivatives alpha-methyl styrene, para-methyl styrene, and t-butyl styrene all are capable of acting as reactive diluents for unsaturated polyester resins. These derivatives offer benefits as a replacement such as having lower vapor pressures. Another monomer similar to styrene in chemical reactivity, while having a higher molecular weight and correspondingly lower vapor pressure, is vinyl toluene. Each one of these styrene monomer substitutes has a clear environmental benefit in that they are not presently regulated by the Environmental Protection Agency (EPA) as a Hazardous Air Pollutant. Title 1 Section 112 (a.) (1.) Clean Air Act Amendment of 1990 defines Hazardous Air Pollutants as ". . . an air pollutant to which no ambient air quality standard is applicable and which in the judgement of the administrator, causes or contributes to air pollution which may reasonably be anticipated to result in an increase in mortality or an increase in serious irreversible or incapacitating reversible illness." One of the drawbacks to these alternatives is their higher cost in comparison to styrene monomer. Another monomer that could serve as a substitute for styrene is methyl methacrylate. However, methyl methacrylate, as is the case with styrene monomer, is considered a Hazardous Air Pollutant and Volatile Organic Compound. Methyl methacrylate also has greater toxicity and the tendency to emit at a significantly higher rate than styrene monomer. Volatile Organic Compounds are defined by the EPA in 40 CFR 51.100 (s) as ". . . any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions." Based almost solely on availability and economics the only applicable monomer for significant consideration is styrene.
Styrene is a flammable liquid that potentially becomes unstable as a result of exposure to excessive heat or open flame. In addition storage recommendations call for the maintaining of an environment below 38.degree. C. (100.degree. F.) and/or avoiding prolonged storage over six (6) months. However, the overriding concerns associated with the use of this monomer are directed towards other areas. The styrene content of the resin is of considerable significance since the emissions thereof are directly proportional to the monomer's content in the resin. Emission levels that result in an increased level of exposure in the workplace are of particular concern.
Another area of deficiency associated with unsaturated polyester resins are their poor physical properties (i.e. low heat distortion temperatures, low tensile strength, high shrinkage levels, low impact strength, etc.) that must be overcome through the incorporation of various organic or inorganic fibers and/or fillers to provide the strength and rigidity and/or reduce the cost required in typical reinforcement applications. Such organic fibers include polyacrylonitrile fibers, pitch-based carbon fibers, aromatic polyamide fibers, liquid crystal polyester fibers or any polymeric fiber that improves the properties of the resin. Inorganic fibers include glass and whiskers, while inorganic fillers include such materials as aluminum trihydrate, calcium carbonate, clay, talc, silica beads, calcium sulfate, ammonium polyphosphate, and the like. The incorporation of the various fibers, however, typically results in an application technique that is labor-intensive and lacking in production controls. The application of the resin/ fiber through the conventional use of "chopper" spray guns results in a substantial amount of material being deposited off of the part and into the surrounding environment. In addition to the cleanup and disposal problems created by the misdirected deposition of the resin and fibers, is the economic impact created by the non-productive use of the materials. To adequately incorporate the glass fibers into the resin matrix it is also necessary to manually roll out the resin saturated glass fibers using hand-held devices that serve to align the fibers and remove the entrapped air voids. The process, in addition to being labor-intensive, exposes the user to increased levels of styrene monomer emissions. To adequately provide the time necessary for the manual alignment of the fibers the resin's cure schedule is extended thereby effectively reducing the optimum obtainable level of productivity. The cure schedule is also subject to the effects of temperature and is by necessity, adjusted for climatic changes through reformulation of the catalyst system. When the temperature fluctuates widely over the course of the day, the cure schedule can be negatively impacted resulting in premature or overly extended gelation. Rapidly reacting resin systems typically generate substantial amounts of heat, which can create internal stresses that are manifested in thermal distortion of the cosmetic layer and /or micro cracking of the reinforcement layer.
The labor element discussed in association with glass-containing thermosetting systems is not a particular concern when addressing present-day gel coat technology. However, gel coat technology is not a stand-alone process and will invariably be accompanied by a reinforcement technology as previously described. The issues typically associated with gel coat technology as a surface or cosmetic layer are resin shrinkage and its inherent structural deficiencies.
Resin shrinkage is prevalent in styrene monomer containing systems owing to the tendency of the monomer to "shrink" during the process of cross-linking. Rates of shrinkage are therefore dependent on the level of styrene monomer in the resin system. Aside from the issue of dimensional tolerances, resin shrinkage degrades the cosmetic appearance of the surface by causing the underlying reinforcement to appear as a faint shadow on the surface, referred to in the literature as "print through". Barrier layer systems have been developed that are applied directly behind the gel coat prior to the application of the reinforcement material(s). These barrier systems have formulations based on syntactic foam, polyurethane or unsaturated polyester-polyurethane hybrid resin systems. These systems are not reinforced and therefore do not contribute to the stiffness of the gel coat. Some systems may be formulated to impart stiffness, however the gel coat remains the limitation to the overall structure since it is the weakest, most brittle part of the laminate. This is inherent to the chemistry of the gel coat since its purpose is cosmetic rather than structural. Cosmetic systems based on polyurethane technology offer the best combination of surface appearance and physical properties, but they are known to be more costly than the existing and prevalent technology based on unsaturated polyesters.
The issues associated with styrene monomer in traditional thermosetting systems have been partially addressed with the development of unsaturated polyester-polyurethane hybrid resin systems. The practice of merging polyurethane and unsaturated polyester technologies to obtain superior properties while reducing styrene monomer levels has been known for decades. These resins are typically tougher than unsaturated polyesters and stronger, stiffer, and less expensive than polyurethanes. These resins fundamentally consist of a hydroxyl-terminated unsaturated polyester polyol, an ethylenically unsaturated monomer such as styrene, and a multifunctional isocyanate. U.S. Pat. No. 4,280,979 teaches the preparation of unsaturated polyester polyols, which can be reacted with a polyisocyanate and a polymerizable ethylenically unsaturated monomer to produce polyurethane/vinyl copolymers. The curing process is a combination of urethane network formation from the reaction of the isocyanate with the reactive end-groups of the unsaturated polyester polyol, and the vinyl addition reaction between the ethylenically unsaturated monomer and unsaturated polyester polyol.
Interpenetrating polymer networks are also known. Technologies of this type consist of a pair of networks, at least one of which has been synthesized and/or crosslinked in the presence of the other. An interpenetrating network can be described as an intimate mixture of two or more distinct crosslinked polymer networks that cannot be physically separated. Interpenetrating polymer networks are also described in U.S. Pat. Nos. 4,923,934, 5,096,640 and 5,382,626. By virtue of its description this technology is grouped with that of hybrid resins particularly when said interpenetrating polymer network(s) are known to include any of the aforementioned monomers.
The art of unsaturated polyester-polyurethane hybrid resins are well known and are described in U.S. Pat. Nos. 4,822,849, 4,892,919 and 5,086,084. Expansion of the technology now includes foam compositions based upon the aforementioned art. Polyester resin foam compositions are described in U.S. Pat. No. 4,460,714, which discloses a low density polyester resin foam made from an admixture of an unsaturated polyester resin, an organic isocyanate compound, a blowing agent, a peroxide curing agent system, a surfactant, and small amounts of an inorganic iron salt. The use of an amine compound to impart nucleation sites to the foam composition is disclosed in U.S. Pat. No. 5,344,852. U.S. Pat. No. 5,302,634 teaches a rigid, lightweight filled foam having voids dispersed in a continuous phase which is formed from a polyester polyol-polyurethane hybrid resin having reinforcing particles selected from fly ash, treated red mud and mixtures thereof dispersed therein. The hybrid resin may form an interpenetrating polymer network with a polyurethane and/or modified hybrid polyurethane resin.
It has been discovered that, in practice, certain properties of the various compositions can be difficult to predict or control. In reference to the '852 patent, blistering was found to sometimes occur at the interface of the rigid foam and the thermoplastic sheet to which it is to adhere, causing weakness and occasional delamination. It was determined that it was possible to establish an association of the blistering and delamination phenomena with zones of high residual monomer. To address these phenomena, U.S. Pat. Nos. 5,447,921 and 5,482,648 teach that the incidence of blistering and delamination are greatly reduced when the monomer is almost completely polymerized in accordance with improvements in the catalyst system. These improvements also contribute to the reduction of styrene monomer that can be released in the form of emissions, but does not completely eliminate these emissions or reduce the monomer content in the system.
Although hybrid resin foam systems exhibit good properties, there are inherent deficiencies associated with the reactive unsaturation in the polyester polyol and the diluent monomer. As noted earlier the drawbacks associated with the stability of unsaturated polyesters are prevalent in the polyester-containing component of the hybrid resin foam. Inhibitors must be added to prevent premature polymerization and to impart storage stability to the polyester polyols. However, the quinone-type inhibitors that are common to the chemistry, are also known to be ineffective in mixtures containing amine-type additives, such as urethane or foam catalysts and chain extenders, in combination with an unsaturated polyol. In order to extend the stability of the hybrid resin system, one skilled in the art must extend the stable life of the polyester polyol component through the incorporation of substituted hindered phenol type compounds with ring substituents that produce activated benzylic hydrogens, nitrophenols (with or without benzylic-type substituents), naphthoquinones, stabilized radical compounds, and mixtures thereof as is taught in U.S. Pat. No. 5,821,296. In addition, to prevent unwanted polymerization of the unsaturation groups, the systems initiators and/or catalysts are added to the isocyanate component which in-turn diminishes the stability of the isocyanate.
Since hybrid resin systems incorporate unsaturated polyester resin technology, it is not surprising to expect it to exhibit similar characteristics in regards to some of its physical properties. Highly filled foam and non-foam systems that result in a rigid laminate tend to exhibit varying degrees of brittleness, and consequently provide little or no impact resistance to reinforce the surface cosmetic layer. Correspondingly, the issue of adhesion particularly to thermoplastic materials is also of considerable concern. In order to obtain adhesion to the thermoplastic cosmetic layer, a skin coat comprised of but not limited to a vinyl ester or adhesion promoting unsaturated polyester resin, epoxy, polyurethane, and the like is typically applied to the cosmetic layer before applying the rigidizing layer. This process in turn slows the rigidizing process and effectively offsets some of the advantages of the faster curing hybrids. Hybrid resin systems as presently formulated exhibited varied degrees of success in achieving adhesion to a select number of thermoplastic substrates. Present day usage of hybrid resin systems is restricted to applications utilizing composite sheet or the incorporation of glass fibers to rigidize monolithic substrates. Applications requiring the incorporation of composite thermoplastic sheet to achieve the necessary strength and rigidity of the combined system ultimately contribute added cost and weight to the fabricated part. When using monolithic sheet the hybrid resin system will typically require the use of a primer system to establish the required level of adhesion. Hybrids also exhibit a tendency to respond to colder temperatures through extended cure times and the necessity of allowing for proper time intervals before adding subsequent layers. Application of additional material prior to the curing of the previous layer typically results in off-gassing and surface blemishes that compromise the structural integrity of the part. The required delay before adding subsequent layers of material contributes to increased cycle times and a reduction in the potential level of productivity. A final deficiency observed in hybrid resin systems, and to a similar degree in unsaturated polyester resins, is the tendency of the organic and/or inorganic filler(s) to separate or settle out from the resin composition. Increases in the viscosity of the filler/resin composition over time have also been observed.
As previously noted, various isocyanate-based rigid foam systems have been used in an attempt to provide reinforcement to thermoformed thermoplastic components. Most often the preferred thermoplastic substrate is two to three times the thickness of the monolithic sheet typically used with unsaturated polyesters. This sheet is also typically a higher-cost composite type employing a cosmetic layer of minimal thickness and a thicker impact resistant layer. Typically the foams of choice are of the closed-cell nature. Although described as rigid foam systems they lack the physical properties of an unsaturated polyester system or of a hybrid resin system unless they have been specifically formulated to include reinforcing fibers or fillers, and have addressed the issue of adhesion to the thermoplastic substrate. U.S. Pat. No. 5,420,169 teaches an invention that produces low-density foams that flow well, are stable and exhibit excellent adhesion to metal and treated thermoplastic substrates. The invention also teaches the preparation of commercially viable foam without the use of chlorofluorocarbons. However, the density range cited (between 1.0 and 2.5 lbs./ft..sup.3) is typically unacceptable as a rigidizing medium. The reference to the elimination of chlorofluorocarbons is also significant in that the need exists to continue to develop foam systems that do not contribute to ozone depletion. With the prior art foams, the chlorofluorocarbons are trapped within the closed cells when the rigid foam is produced. Thus, the chlorofluorocarbons produce a detrimental environmental effect both when the foam is produced and later, when it degrades and the closed cells release the entrapped chlorofluorocarbons. Lower density foams such as those referenced above, are known to exhibit the tendency to absorb water either through immersion, contact or through environments with prolonged high humidity. Environments having high humidity and temperature are known to create application issues that require specialized care and contribute to the cost and productivity of using these products.